When embarking on a journey in the Pokémon games, players quickly learn that in order for both Pokémon and trainer to reach their full potential, they must work together as a team. Players catch their teammates from the wild with the goal of raising and evolving powerful Pokémon to take on the gym obstacles and potential baddies of the region. However, there is plenty of evidence in the Pokémon games to suggest that Pokémon don't need their trainers to evolve, and are perfectly capable of doing so alone in the wild.
It has been shown in the Pokémon Anime time and again that wild Pokémon develop without a trainer's help. In episode 798 of Pokémon X&Y, "A Battle of Ariel Mobility," Ash and his friends encounter a situation where a wild Fletchinder and Dedenne have an altercation over a berry. The standoff shows that Pokémon often have encounters in the wild which would presumably allow them to gain experience like a trainer-led battle would. Fans also learn Pokémon can establish a natural order between themselves in a location during the Pokémon Journeys Episode 23, "Panic in the Park." Ash and Goh's caught Pokémon must learn how to get along together in the park where they live once caught, with minimal help from their trainers, giving Ash and Goh insight on how Pokémon behave in the wild.
When it comes to evolutionary items, the Pokémon games themselves put that to rest. Players can find evolutionary items and stones laying around in grass, caves, behind buildings, or fished from bodies of water. It wouldn't be hard for a wild Pokémon to come across the needed item by chance and have the opportunity to evolve. It has also been shown in the Anime, by Pokémon like Pikachu, that Pokémon understand what evolutionary items are and can make a distinct choice on if they want to use the item or not.
Pokémon Can Grow In The Wild Without A Trainer
The most compelling evidence that can be offered to fans for wild Pokémon evolution, however, is present from the first generation games forward. Players can encounter evolved forms of Pokémon in the wild. In earlier games, these evolved forms often appear in the later part of the regions, where encounters are stronger. In Pokémon Sword & Shield however, the Wild Area erases that, allowing evolved forms to be seen from early in the region. With evolved Pokémon roaming the wild, it can be assumed that they must have a way to grow and change without trainer interference.
The only form of Pokémon evolution that are likely barred from the wild is trade-triggered forms. Pokémon like Haunter and Pumpkaboo can't evolve unless they are traded from one trainer to another. Because trading isn't possible in the wild, it would make sense that Gengar and Gourgeist can't exist outside of a trainer-Pokémon relationship. While training and battling is an critical part of a player's Pokémon journey, it is humbling to know that even without trainers, the world of Pokémon would continue to grow, change, and evolve on its own.